Volcano Hazards in the San Salvador
Region, El Salvador

Summary

Concepción is one of Nicaragua’s highest and
most active volcanoes. The symmetrical
cone occupies the northeastern half of a dumbbellshaped
island called Isla Ometepa. The dormant
volcano, Maderas, occupies the southwest half of
the island. A narrow isthmus connects
Concepción and Maderas volcanoes. Concepción
volcano towers more than 1600 m above Lake
Nicaragua and is within 5 to 10 km of
several small towns situated on its aprons at or
near the shoreline. These towns have a combined
population of nearly 5,000. The volcano has
frequently produced debris flows (watery flows of
mud, rock, and debris—also known as lahars when
they occur on a volcano) that could inundate these
nearby populated areas.

Concepción volcano has erupted more than 25
times in the last 120 years. Its first recorded
activity was in AD 1883. Eruptions in the past
century, most of which have originated from a
small summit crater, comprise moderate
explosions, ash that falls out of eruption plumes
(called tephra), and occasional lava flows. Near
the summit area, there are accumulations of rock
that were emplaced hot (pyroclastic deposits),
most of which were hot enough to stick together
during deposition (a process called welding). These pyroclastic rocks are rather weak, and tend
to break apart easily. The loose volcanic rock
remobilizes during heavy rain to form lahars.
Volcanic explosions have produced blankets of
tephra that are distributed downwind, which on Isla
Ometepe is mostly to the west. Older deposits at
the west end of the island that are up to 1 m thick
indicate larger explosive events have happened at
Concepción volcano in prehistoric time. Like
pyroclastic-flow deposits, loose tephra on the steep
slopes of the volcano provides source material that
heavy rainstorms and earthquakes can mobilize to
trigger debris flow.

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